Re-triggering failed translations
Whenever a translation fails (due to the API going offline, having an invalid API key, running out of credits, etc), you can re-trigger translating that entry and language only, and avoid spending API credits for translations that were successful.
You will notice that an entry/language has failed translating as these are highlighted with a yellow background on the Polylang edit icon in the list pages:

You can also filter to show only entries with failed translations (in the Posts and Media list pages only):

Filtering these entries allows you to easily re-run the translation for all of them at once.

Finally, re-run the failed translations by executing the Gato Translate (Custom) bulk action selecting the Process failed translations only option:

If using WP-CLI, you can re-trigger failed translations using the --process-failed parameter.